The present invention relates to a camera which utilizes the variation of contrast of an object as the focus position of the photographing lens of the camera is changed to automatically detect the position of correct focus.
An example of a focus detecting system is disclosed by Japanese Laid-Open Patent Application No. 64711/1982, filed by the same applicant. In this system, two contrast detecting elements are provided, disposed on opposite sides of a position equivalent to the position of the focal plane with respect to the lens system equidistantly spaced from that position. Contrast signals outputted by the contrast detecting elements are used to detect the position of correct focus. However, that system is rather troublesome because, whenever the lens is changed to one having a different maximum (full-aperture) f-number, it is necessary to change the focus detecting algorithm employed by operating a mechanical switch. This problem may be solved by employing some device for automatically changing the algorithm of the focusing detecting circuit when the lens is changed. However, such a device is complex and not practical.
This situation will be described in more detail with reference to FIGS. 1A through 1C. In FIG. 1A, the horizontal axis indicates an amount of extension (le) of a lens (related to the position of the focusing ring), while the vertical axis indicates the magnitude of a contrast signal produced as the amount of extension is varied. More specifically, contrast detecting elements SA and SB (not shown) are arranged at respective positions a and b corresponding to two different amounts of lens extension, one on either side of the plane of the film surface. The amounts of light received by the two detecting elements SA and SB are equal. Therefore, when the focused position coincides with either of the detecting elements' position, the respective contrast signal is a maximum. Each of the contrast signals falls off symmetrically on both sides of this position. The contrast signals 1a and 1b have similar distributions. Accordingly, the position 2 of the intersection of the distribution curves of the contrast signals 1a and 1b is equidistant from the two positions a and b and represents the correctly focused position, that is, when the difference between the contrast signals 1a and 1b becomes zero, a correctly focused position is obtained.
To implement this focus detecting scheme, a difference signal 1c between the contrast signals 1a and 1b is produced as shown in FIG. 1B. As indicated in FIG. 1A, an effective focus range limit level 3 is set for the contrast signals 1a and 1b, and a focus indicating signal is outputted when the amount of lens extension le is in a range of b.sub.1 to a.sub.1 in which the values of the contrast signals 1a and 1b are both higher than the limit level 3. In addition, the absolute value 1d of the difference signal 1c is obtained as shown in FIG. 1C. When the signal 1d is smaller than a focus detecting threshold level 4 and is in the range b.sub.1 to a.sub.1, that is, when the amount of lens extension is in a range 5, a focus indicating signal is outputted indicating that the present lens position is acceptable. In the case where the signal 1d is larger than the threshold level 4, when the signal 1c (FIG. 1B) is positive and larger than the threshold level 4, a signal representing a front focused state is outputted, and when the signal 1c is negative and smaller than a level 4' (which is symmetric about the X axis with respect to the threshold level 4), a signal indicating a rear focused state is outputted. Furthermore, when the signal is out of the range 5 and smaller than a level 6, that is, when the amount of lens extension is in a range 6a or 6b, a signal indicating the fact that a focused position cannot be detected is outputted. Accordingly, the signal indicating the front focused state is outputted when the amount of lens extension is in a range 7a, and the signal indicating the rear focused state is outputted when the amount of lens extension is in a range 7b. In order for the focus indicating range 5 to be independent of the contrast of the object, the threshold level 4 is made proportional to the sum of the contrast signals 1a and 1b.
In general, as the full-aperture f-number of a photographing lens increases, the depth of field increases. Therefore, as shown in FIG. 2D, contrast signals 1a" and 1b" produced in the case where the full-aperture f-number is large, have wider distributions than contrast signals 1a' and 1b' (FIG. 2A) produced when the f-number is small. The peak values of the contrast signals do not change when the full-aperture f-number is changed. Therefore, the focus detecting threshold level 4b for the case of a large full-aperture f-number indicated in FIG. 2E is substantially equal to that 4a of FIG. 2B for which the full-aperture f-number is small. Accordingly, a focus indicating range 5b defined by the intersections of the threshold level 4b and an absolute value signal 1d", similar to that of FIG. 1C, is much wider than a focus indicating range 5a for an absolute value signal 1d' in the case where the full-aperture f-number is small. As a result, the depth of field of the photographing lens may be exceeded in the former case. In order to overcome this difficulty, it is necessary to set the focus detecting threshold levels 4a and 4b in accordance with the full-aperture f-number so that the widths of the focus indicating ranges 5a and 5b correspond with the depth of field of the photographing lens. In order to meet this requirement, it is necessary to provide a switching mechanism on the camera which is set in accordance with the full-aperture f-number of the photographing lens whenever the photographing lens is changed.
In more detail, if the effective focus range limit levels provided for the cases of small and large full-aperture f-numbers are fixed such that the levels 3a and 3b are equal to each other, a focus indicating range b.sub.1 ' to a.sub.1 ' can be defined for the contrast signals 1a' and 1b' produced in the case of a small full-aperture f-number as shown in FIG. 2A, while for the contrast signals 1a" and 1b" produced in the case of a large full-aperture f-number, the focus indicating range is substantially unlimited, and accordingly a false focus indicating signal may be produced in the ranges indicated by shading in FIG. 2F. In the case illustrated in FIG. 2C where the full-aperture f-number is small, focusing is indicated only in the range b.sub.1 ' to a.sub.1 ', and therefore no false focus indicating signal is produced.
The case will now be considered where, in order to make the effective focus range limit level suitable for the situation in which the full-aperture f-number is large, the effective focus range limit level 3c is made relatively large for contrast signals 1a.sub.1 and 1b.sub.1 to thus obtain a range b.sub.1 " to a.sub.1 " as indicated in FIG. 3A. In this case, contrast signals 1a.sub.2 and 1b.sub.2 for an object of relatively low contrast are smaller than an effective focus range limit level 3d equivalent to the above-described effective focus range limit level as shown in FIG. 3D. Accordingly, as shown in FIG. 3E, the focus indicating signal is inhibited in a focus indicating range 5d defined by the intersections of a focus threshold level 4d and an absolute value signal 1d.sub.2 similar to that of FIG. 1C. The inhibited output is indicated by a dashed line in FIG. 3F. In the case where the contrast is high, as shown in FIGS. 3B and 3C, a focus indicating range 5c defined by an absolute value signal 1d.sub.1 and a focus threshold level 4c indicates the fact that the focus indicating signal in the range b.sub.1 " to a.sub.1 " is outputted in the range indicated by the solid line.
As is apparent from the description of FIGS. 2A to 3F, if the effective focus range limit level is fixed, then a false focusing signal may be produced in some instances. In order to eliminate this difficulty, it is necessary to determine the threshold levels 4a and 4b in accordance with the full-aperture f-number of the lens. Also, it is also necessary to set the effective focus range limit level to a value suitable for each full-aperture f-number. If the conventional approach as discussed above is employed to meet these requirements, the resulting system is accompanied by a troublesome switching operation.